A few years ago I designed a typeface called Half Cut Gothic and made it available for free at Dafont and FontSpace, but now a marketing firm here in Canada has contacted me regarding "ownership purchase". Considering it has been available for free for so long, how is it possible to change that? If copyright-wise this is even possible, how much do you think would be a fair price? ...Anything else I should be wary of?

They don't seem too concerned about the freeware part. Quote:

"Since we have already used the font in some of our pieces, we'd like to continue using it, rather than creating a new font. We are ok knowing that people may have purchased the font already - but would be curious to know moving forward if we could purchase the ownership for the font."

1) Pull it off the channels you have control over.
2) Warn them that third parties might still be distributing it, not just using it (which even happens to non-free fonts); and if they mind, they would have to be the ones trying to get them to stop, not you.
3) The price should depend on a host of things, some difficult to nail down. Looking at the font, but not knowing anything about your customer, shooting from the hip I would say $3000 (leaving room for negotiation).

If you're the type to worry that an eventual lawsuit might threaten your assets, do talk to a lawyer. But to me this case doesn't seem tricky enough to waste the money. And remember, you have to keep the lawyer happy too...